Why are Americans so overweight?

Of all the countries of the world, Americans are the fattest. Why? Although the cause is probably many of these answers, please say what you consider to be the MAIN cause.

Note 1: Yes of course some Americans aren't fat. This is referring to the population in general.

Note 2: Current statistics suggest that nearly 60% of Americans are significantly overweight - a higher figure than the rest of the world, with the exception of a few small Pacific islands.